Ferrite adjustable kicker magnet for extracting beams of charged particles



March 26, 1968 I E. B. FORSYTH 3,375,452

- FERRITE ADJUSTABLE KICKER MAGNET FOR EXTRACTING BEAMS OF CHARGED PARTICLES Filed Jan. 13, 1967 5 Sheets-Sheet l 35 M l9 l7 I8 37 29 4 25 33 1 3| l8 T J j U 40 a3 23 LJ 39 I5 27 M 4| Fly. 1

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b ERIC 8. FORSYTH flan? March 26, 1968 E. B. FORSYTH 3,375,452 FBRRITE ADJUSTABLE KICKER MAGNET FOR EXTRACTING BEAMS OF CHARGED PARTICLES Filed Jan. 13, 1967 5 Sheets-Sheet 2 INVENTOR.

BY ERIC B. FORSYTH a I flM 4 W Hg. 4

.; ite States atent O 3,375,452 FERRITE ADJUSTABLE KICKER MAGNET FOR EXTRACTIN G BEAMS OF CHARGED PARTICLES Eric B. Forsyth, Brookhaven, N.Y., assignor to The United States of America as represented by the United States Atomic Energy Commission Filed Jan. 13, 1967, Ser. No. 609,730 7 Claims. (Cl. 328235) ABSTRACT OF THE DISCLOSURE This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION l. Field of the invention In high energy physics a need exists for a device capable of extracting beams of charged particles from the endless vacuum chamber of high energy accelerators. These beams vary over a wide range of betatron oscillation amplitudes and beam diameters from low beam momenta at injection to high beam momenta at ejection and are particularly of interest over an average variation of betatron oscillation amplitudes and beam diameters from about two inches to about one-half inch or less.

2. Description of the prior art Stationary, constant aperture kicker magnets known heretofore do not operate efficiently at such variable beam diameters since they have been subject to severe flux degradation proportional to the square of the flux density times the aperture volume i.e. B V. Thus they have been subject to the disadvantage of not being able to extract all the beam particles in a small diameter beam at high beam momenta whereby these kickers have produced extremely dangerous or undesirable radiation levels due to scattering in the kicker magnets or in their associated equipment, such as the septum magnets for deflecting the beam downstream from the kicker magnet. Others, such as the movable magnets disclosed in U.S. Patent 3,128,406, which have required the manufacture and assembly of movable or flexible electrical conductors, have been subject to frequent and expensive breakdown adjustment or repair of these conductors due to wear and fatigue. Additionally, it has been advantageous to provide a practical, simple and trouble-free kicker magnet that maintains the integrity of the vacuum chamber for the charged particle beam.

It is an object of this invention, therefore, efliciently to provide a practical and economical apparatus and method for the deflection of small diameter beams of charged particles and the transmittal of large diameter beams of charged particles by providing stationary and movable, magnetically susceptible, non-conducting members that form a variable size aperture for accommodating large and small beam diameters;

It is another object to eject charged particles from a Patented Mar. 26, M363 beam travelling along an endless equilibrium axis in such a way to avoid the use of movable electrical conductors;

It is another object to provide a variable aperture kicker magnet that will cause the least scattering interference with a beam of charged particles;

It is a further object to eject charged particles from a beam travelling along an endless equilibrium axisin a closed vacuum chamber in such a way as to avoid introducing vacuum leaks into the chamber;

It is a further object to minimize the stored energy in the aperture of a kicker magnet by providing a small vertical clearance when the magnet is energized and a larger vertical clearance when the magnet is not energized;

It is a still further object to reduce the weight of kicker magnets while increasing their kicking ability.

SUMMARY OF THE INVENTION This invention provides selectively energized stationary electrical conductors, stationary magnetically susceptible members and movable magnetically susceptible members that selectively separate to form a large or open-ended aperture of predetermined dimensions for receiving a large diameter beam and selectively close to form a small cross-section rectangular shaped aperture of predetermined dimensions in timed sequence with the energization of said conductors for efiiciently ejecting a small diameter beam, means for biasing the movable members to form the large or open-ended rectangular shaped aperture, a chamber means having bellows for maintaining a completely vacuum tight seal between said moveable members and said biasing means while accommodating relative movement between said chamber and said biasing means, and means for selectively limiting the movement of said moveable members to provide said predetermined dimensions in said open-ended aperture and said gap. With the proper selection and arrangement of components, as described in more detail hereinafter, the desired troublefree ejection and vacuum integrity are achieved.

The above and further novel features and objects of the invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings where like elements are referenced alike:

FIGURE 1 is a partial schematic view of an accelerator ring incorporating the kicker system of this invention;

FIGURE 2 is a partial circuit diagram of the pulse forming network and discharge circuitry for the fast kicker of FIGURE 1;

FIGURE 3 is a partial circuit diagram of the conductors of the fast kicker of FIGURE 1;

FIGURE 4 is a partial three dimensional view of an embodiment of this invention;

FIGURE 5 is a partial three dimensional view of another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is known that charged particle beams can be accelerated from low momenta to high momenta in the vacuum chamber of an endless accelerator ring. One such accelerator is shown and discussed in U.S. Patent 2,882,396 by E. D. Courant et al., the description and figures of which contemplate a beam travelling in an endless evacuated tube at low momenta and thereafter at higher momenta in longitudinally extending bunches close to the speed of light. The vacuum chamber has an elliptical cross-section, and in one practicalsystem thereof in the Brookhaven National Laboratory AGS, the tube has a vertical minor axis of about 2.7 inches and a horizontal major axis of about 6 inches to accommodate an injected beam having an average diameter of about two inches.

Focusing of the beam is based on the strong focusing or alternating gradient principle, which confines the beam along an endless equilibrium axis or orbit in the tube so that the beam does not hit the sides of the tube as the beam oscillates with a preselected small betatron oscillation amplitude along the axis. In the Brookhaven National Laboratory Alternating Gradient Synchrotron, whose principles and operation are described in the cited patent, the betatron oscillation wave length is about 300' feet and the betatron oscillation frequency is from 13 megacycles per second from injection to full beam energy, when the beam diameter is about .25 inch on the average. The invention hereinafter described ejects a beam of this type from an accelerator of the above-mentioned variety.

In order to explain how the method and apparatus of this invention accomplish this ejection, reference is made to FIGURE 1, wherein is illustrated an endless equilibrium orbit axis 13 along which a beam 15 of high energy particles travels in bunches 17 detected by suitable beam induction probes 18. Orientation of the figure is shown by the arrow 19, which is disposed radially outwardly from a point (not shown) corresponding to the center of the beam orbit in the accelerator 21. Arrow 23, represents the direction of the beam travel.

It is desirable to provide a fast, single-turn extracted beam of protons up to about 33 bev. energy or more with high efiiciency, in beams having densities up to particles or more. To this end three pulsed magnets 25, 27, and 29 couple to the beam by coherent betatron oscillations and the magnets have their own vacuum enclosures 31, 33 and 35 that connect with the main vacuum tube 37 of accelerator 21. Magnets 25 and 27 have septum plates 39 and 40 dividing their magnetic fields into areas of high magnetic field and almost zero field. The fields in these magnets develop relatively slowly, being fully established immediately prior to ejection from tube 37 into tube 41. Suitable means M and M withdraw both septum magnets before injection to avoid restricting the aperture size of the accelerator 21, which as a practical matter corresponds with the size of the tube 37, and ram the entire septum magnets inwardly toward the beam axis 13 after the beam diameter shrinks due to acceleration. The power supply (not shown) for the septum magnet 25, comprises a closed loop, current regulating servo, whose regulating element is a bank of 200 power transistors connected in parallel. The loop compares the voltage across a shunt with a reference and amplifies the error signal. The power supply for the septum magnets is located about 80 feet from the magnet, outside the accelerator tunnel, to prevent deterioration of the power transistors due to. radiation damage. The power supply for magnet 27 the ejector magnet) is a 16,000 mf. capacitor bank with ignitron switch tubes.

Before ejection, normal acceleration occurs in the accelerator 21, partially shown for ease of explanation, as the beam 15 lies on the zero field side of the septa 39 and 40. The magnet 29 of this invention, receives pulses from source 43, at a predetermined fixed time after injection of beam 15 into accelerator 21 from a suitable source (not shown) whereby this magnets field has a rise time shorter than the time between adjacent proton bunches 17 in the accelerator 21. Thus, the pulsed field, properly synchronized to the bunch position, deflects the beam 15 from the main vacuum tube 37 of the accelerator 21 into the high field areas of the septum magnets and out of the accelerator 21 through the transport tube 41, which connects with the main beam tube 37.

The power supply 43 for the fast pulsed magnet 29, comprises low-impedance, high-voltage pulse-forming networks (PFNs) switched by thyratrons 44, as illustrated in FIGURE 2. The pulsed field requirements or this fast kicker correspond to the current waveform associated with a line-type modulator. Thus the current must flow in the kicker in series with matched terminating resistors and the inductance of the kicker must not cause a radical change in the discharge current waveform. To this end the kicker magnet system 29 has several like sections A, B, C and D that are electrically in parallel and magnetically in series as illustrated in FIGURE 3. Each section is alike.

Referring now to FIG. 4, a practical embodiment of one section, for example section A, of the fast kicker 29 of this invention, comprises rectangular ferrite blocks 45 and 45' and 46 and 46. The blocks 45 and 45' are stationary and have dipole magnetic particle deflecting conductors 47 and 49 thereon, which are insulated from the blocks 45 and 45' by insulators 51 and 53 and straddle the opposite sides of the axis 13 and beam 15. Ferrite blocks 46 and 46' straddle the top and bottom of axis 13 and have provision for a first movement that forms an open ended aperture 61 inside the blocks for a beam 15 of large cross-section. Provision is also made for a second movement to provide a small cross-section aperture 61 with a width illustrated by arrow A and a height illustrated by arrow B .to accommodate the small cross-section beam 15 after acceleration in accelerator 21. The side portion of the blocks, i.e. blocks 45 and 45, remain stationary, thus providing a fixed uniform dimension A in aperture 51, while another portion of the blocks move relatively apart to form an increased uniform cross-section aperture 61 at least as big as the vertical distance across the injected large cross-section beam wherein the dimension Y represents the maximum height of the beam 15. The movement provided is such as selectively to straddle the large and small cross-section beams 15 with an equal minimum small clearance from the maximum vertical beam envelope dimensions of the accelerated and injected beams 15 to the top and bottom blocks 46 and 46 so as to prevent scattering of the particles from the beam 15.

Energizing of the conductors 47 and 49 produces magnetic field lines, whose direction is illustrated by arrows 63 and 65, in the small cross-section aperture 61 at right angles to the beam direction 23, and these field lines pass through the top and bottom blocks 46 and 46' and the side blocks 45 and 45' in continuous paths to create a high, concentrated flux in the small cross-section aperture 61 whereby the beam deflects a sufiicient amount for ejection from the accelerator 21.

In this regard, the energization of the conductors 47 and 49 is in opposite directions in series whereby the magnetic field lines pass through the side blocks 45 and 45' in opposite directions substantially parallel with the direction of the field lines in the aperture 61 and through the top and bottom blocks substantially at right angles to the direction of the field lines in the aperture 61. Moreover, the directions 63 and 65 of the field lines relative to each other are opposite and rotate, with the top and bottom blocks 46 and 46' providing the respective return paths for transmitting the magnetic field lines across the aperture 61 in continuous paths. Thus, at the intersection of a vertical plane on the axis 13 and normal to the top and bottom blocks 46 and 46' the field lines in aperture 61 divide from each other into the top block 46 and come together in the bottom block 46 with the field line direction 63 being counter-clockwise on one side of the plane and the field line direction 65 being clockwise on the other side of the plane.

In the embodiment, illustrated in FIGURE 4, the top and bottom blocks 46 and 46' move relatively up and down toward and away from each other. Advantageously, when they move toward each other they form horizontal spaces adjacent the top 73 and bottom 75 of side blocks 45 and 45' between the top and bottom blocks and their adjacent side blocks 45 and 45' transverse to the return paths of the magnetic field lines in the ferrite blocks as the'field lines progress into and out of the top block 46 and the bottom block 46 through the intermediary of the side blocks 45 and 45' and the aperture 61. These spaces which prevent shock damage to the blocks in moving together suddenly, are small relative to the height B or width A of the aperture 61 to minimize flux degradation in these spaces adjacent surfaces 73 and 75. Round portions p and p of rack R connect with block 46 through vacuum seal E, such as bellows E in the side of enclosure 35 for the blocks 45, 45, 46 and 46 and the conductors 47 and 49, said enclosure 35 connecting in a closed system with tube 37.

In this embodiment, source 76 energizes actuators 77 and 77 upon command through lead 0 from a control 0 a predetermined time after injection of beam 15 into accelerator 21 thereby to energize the actuators in the proper direction to bias the blocks 46 and 46 selectively to increase or decrease the cross-sectional area of aperture 61 to the desired size in the proper sequence while control 0 connects with source S through lead S for the energization of conductors 47 and 49 in the proper sequence.

For ease of explanation, the top actuator 77 will be described, although it is like the bottom actuator 77 for block 46'. This actuator, comprises linear motor 79 that is energized in one direction to move the block connected therewith inwardly and energized oppositely to move the block connected therewith outwardly. The position of the blocks is monitored by rack R, pinion R and drive 84 that is connected between pinion R and potentiometer 85. Portion p of shaft R extends to seal E and portion p of shaft R extends from the inside of seal E to the block 46 so that the block 46 is moved by shaft R.

Limit switches 86 and 87 are actuated by the movement of tabs 88 and 89 on shaft R electrically to brake the movement of motor 79 in each direction to limit the movement of shaft R at the desired predetermined positions. Thus the movement of shaft R is limited in one direction by one or the other of the limit switches.

In one sequence for an aperture 61 having a length of 60" in kicker 29, the field rise due to energization of conductors 47 and 49 begins just after the downstream end of one particle bunch 17 leaves the kicker 29 and the field strength rises to its maximum before the beginning of the next adjacent bunch 17 enters the aperture 61. This requires a rise time of 200 Nano-seconds. The top actuator 77 and the like bottom actuator 77 are simultaneously actuated from control 0 through lead 0 selectively and sequentially to move the top and bottom blocks 46 and 46' toward each other. Thereafter, the control 0 energizes conductors 47 and 49 to eject the small diameter beam.

The ferrite blocks are ground high-resistivity nickelzinc ferrite with initial permeability of at least 500, a saturation flux density of 2900 gauss, and a maximum flux density of 1500 gauss in the ferrite. The insulators 51 and 53 which insulate the conductors 47 and 49 from the ferrite blocks 45 and 45' comprise thin rectangular shaped channels of glass having opposite flanges 91 and 93 that prevent arc-over from the conductors to the top and bottom blocks 46 and 46.

In another embodiment of section A of kicker 29, illustrated in FIGURE 5, the actuator operates without gaps transverse to the magnetic field lines. To this end the ferrite block movements provide two horizontal, oppositely moving top and bottom blocks meeting across gaps coinciding with a vertical median plane through the kicker 29 on the equilibrium axis 13. The side blocks 45 and 45 abut with the bottom blocks 95 and 95 and the top blocks 96 and 96' in parallel planes. The side blocks meet the top and bottom blocks in sliding contact in parallel planes. Like actuators 77 are provided for the top and bottom blocks, but for ease of explanation the top actuators will be described.

The top actuator 77 comprises linear motors 79 and 79" that bias a split top block, comprising two sections or top slabs 96 and 96, horizontally relatively oppositely relative to bellows E. These bellows provide for the movement of the respective drive shafts 97 and 99 through the side walls 101 and 103 of vacuum enclosure 35. These bellows are effective since they accommodate a predetermined lateral movement in opposite directions with ease, simplicity and reliability to provide a total opening of two inches across gap 105 when the blocks 96 and 96 are biased apart. Control 0 energizes these motors '79 and 79" through lead 0' selectively to move the shafts 97 and 99 inwardly and outwardly at the appropriate time, with the desired force, speed and linear movement. Suitable limit switches 86 and 87, like those described above, limit the movement of the top and bottom blocks to predetermined dimensions by electrically braking the motors of each actuator at the proper time.

In review of the operation of the described system incorporating the fast, high rise time kicker 29 of this invention, the beam 15 initially has a large diameter at injection. An open ended aperture 61 formed by kicker 29 receives this beam with a suitable clearance x, e.g. .5 inch to 1 inch. After acceleration, the aperture 61 of this kicker closes to a small cross-sectional area with a like clearance tolerance. in FIGURE 5, the dotted line 141 represents the portion of the large diameter beam 15 that would hit the closed blocks 96 and 96. Accordingly, the blocks open sufliciently to prevent interference with this portion of the beam 15.

After a small diameter bunch leaves the aperture 61, the blocks move together to form a small rectangular cross-section aperture 61 with a small clearance tolerance and the conductors are energized so that they produce their peak field before the next bunch enters this small aperture 61. Meanwhile, the septum magnets 25 and 27 are rammed inwardly whereby the field in kicker 29 deflects the beam 15 into these septum magnets and out of the accelerator 21 through tube 41. Then the septum magnets withdraw and the kicker 29 opens up to receive the next injected large diameter beam for the beginning of the next cycle.

This invention has the advantage of providing a variable aperture fast kicker for efiiciently, safely and substantially completely extracting an entire small cross-section beam from a high energy endless accelerator ring, and for receiving a large cross-section beam at injection. This kicker eliminates the heretofore known flexible or moveable electrical connections as well as the overall magnet movement required heretofore. Moreover, simple, commercially available and effective vacuum connections and accurate predetermined dimensions are provided whereby the kicker of this invention is easy to fabricate and operate, light in weight, and dependably produces higher magnetic deflecting fluxes and more complete extraction than were possible heretofore.

What is claimed is:

1. In combination with particle extracting apparatus for use with a beam of high energy charged particles traveling in a closed vacuum chamber along an endless equilibrium axis, comprising opposite electrical conducting means for producing magnetic field lines of force in said chamber, and ferrite blocks with which said conductors form an aperture for said charged particle beam and continuous paths for said field lines that concentrate said field across said axis at right angles thereto, the improvement comprising biasing means, and chamber means connected to said biasing means for maintaining the vacuum integrity of said chamber, said biasing means connecting with a portion of said blocks through said chamber means for biasing said portion of said blocks relatively to form a large cross-section aperture for receiving large diameter, low momenta beams at injection into said chamber and for moving a first portion of said blocks relatively to form a small cross-section aperture for receiving and efficiently ejecting small diameter beams at high momenta, said conductors and said remaining ofsaid ferrite blocks being stationary, and said first portion of said ferrite blocks being moveable relative to said conductors and stationary blocks.

2. The invention of claim 1 in which said blocks form a rectangular aperture bounded by opposite vertical first blocks and opposite horizontal second blocks, said first blocks being stationary adjacent said conductors, and said second blocks being moveaable relative to each other and relative to said first blocks.

3. The invention of claim 1 in which said moveable blocks are relatively moveable vertically.

4. The invention of claim 1 in which said moveable blocks are relatively moveable, horizontally, said biasing means, comprises linear induction motors, and said chamber means having a vacuum tight bellows connected between said linear induction motors and said chamber means.

5. The invention of claim 1 having means for selectively energizing said conductors and moving said moveable ferrite blocks in a timed sequence whereby said magnetic field lines of force are bent to reach their maximum flux density in said aperture when said aperture has its smallest cross-sectional area.

6. The invention of claim 1 having means for selectively stopping said ferrite block movement to provide a small space between said moveable andfixed blocks that is smaller than the largest dimension of said aperture cross-section.

UNITED STATES PATENTS 2,599,188 6/1952 Livingston 31362 3,128,405 4/1964 Lambertson 31362 3,255,369 6/1966 Jacquet 32 8-235 X 3,284,744 11/1966 Danby et al. 328235 X: 3,303,426 2/1967 Beth 3'28235 3,323,088 5/1967 Lambertson 328235 X JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner. 

